3.5.4 Resuming sessions

The TLS handshake process performs expensive calculations
and a busy server might easily be put under load. To
reduce the load, session resumption may be used. This
is a feature of the TLS protocol which allows a
client to connect to a server after a successful handshake, without
the expensive calculations. This is achieved by re-using the previously
established keys, meaning the server needs to store the state of established
connections (unless session tickets are used – Session tickets).